extracting bending moment loads from a CQUAD shaft mesh in NASTRAN ?

[sumner1979]

I've been experiencing a problem trying to extract the bending moment load on a CQUAD4 shaft/cantilevered beam mesh. I know what the answer should be to begin with. I began by checking the shear loading on the shaft by checking the Grid Point Force Balance results. These turned out ok. I then went to checking the bending moment at the same location as I took the shear. This was way off. Nothing even reasonable. I then experimented with writing out the element forces to get this load, that didn't work either.

For the shear loads, I took the sum of the grid point forces contributions from the elements on one side of a ring of grids on the shaft. This worked out well, and I presummed that I would/should do the same for the bending moments loads... any ideas ? I did this problem with CBeam and CBar elements and the results were very good. But in the jump to the QUAD mesh.. your guess is as good as mine at this point.

Let me also note, when I loaded the ring of grids on the shaft on the free end through the use of an RBE3, and then on the fixed end I put RBE2's between the ring of grids on the shaft and an added center node where the BC was placed, I obtained the right moment reaction in the Grid Point Force Balance.

Any help would be much appreciated.

 

[fkmeyers]

When you looked at the moments, were these the GP Force moments or the element results moments?

The GP Force loads will always be correct because you must have equilibrium at the nodes in an FE static analysis. When I say correct I should say balanced with the applied external loads. GP Force loads come from the element stiffness matrix x the displacements at the nodes. Now the element based forces and moments are derived from stresses at the top and bottom of the shell at the integration points and then averaged for the center value and extrapolated for the corner values. These results get better as the mesh gets finer. You are not talking about these bending moments, correct?

 

[sumner1979]

I tried both approaches.

I originally thought that if I took a ring of grids on the perimeter of the CQUADS for the beam, and from this ring of grids, I summed the Grid point force moment contributions from the elements on either the left hand side or right hand side, I'd end up with the bending moment on that section. I took this same approach with the shear force on that section of the beam and it worked. I also took this approach with a Cbar and Cbeam mesh, and using the grid point force balance it worked as well.

When I tried looking at the element forces, it was more to try to explain the results that I got. When I saw that the moments I had with the grid point forces balance were so small, I figured I'd try another approach. That didn't seem to work either...:^)

 

[fkmeyers]

If you have a tube made of shell elements I would expect any GP Moment values to be small. Most of the action in the shell elements will be in-plane to the element. There is very little relative bending unless the tube has a very small diameter to thickness ratio. We do rocket structures and use this approach as well. We deal with 8-16 foot diameters and 0.1 inch thicknesses. Looking at just the GP Forces and not the moments (which should be negligible) you should be able to sum forces in the axial direction to zero for a shear load on the tube. The forces in the shear direction should equal that of the applied shear load. Summing the moments using just the force components should come out close to the shear load x the axial distance to the cut section where the loads are.